The present invention relates to a hinge structure for rotatably supporting a mirror surface of a micromirror device for scanning a beam.
Micromirror devices have widely been used in various technical fields like optical switches for communication, measuring instruments, scanners, etc. For example, in a micromirror device of a capacitance type, a mirror surface for scanning an incident beam is rotatably supported by a hinge structure having elasticity and a plurality of electrodes are arranged on a substrate placed under the mirror surface. By applying voltage to a proper electrode, electrostatic attraction is caused between the electrode and the mirror surface and thereby the mirror surface is tilted in a desired direction. In recent years, micromirror devices are being required to widen their beam scan range by tilting the mirror surface by a larger tilt angle. For this purpose, the hinge structure is required to improve its spring performance, that is, to bend or twist more flexibly. Under the circumstances, not only simple rod-like hinge structures but also various types of hinge structures have recently been proposed as disclosed in Japanese Patent Provisional Publication No. 2003-29172, for example.
A study of conventional hinge structures employing the finite element method, etc. has proved that the spring performance changes inversely proportional to the size of the hinge structure. For example, in a hinge structure of the so-called continuous z-fold type (thin material alternately folded in directions orthogonal to an axis) described in the publication, the size of the hinge structure measured in a direction orthogonal to the rotation axis of the mirror surface has to be increased in order to improve the spring performance. However, designing the hinge structure too large can cause a drop in the strength of the whole mirror layer in which the mirror surface is formed.